This invention relates generally to a development apparatus for ionographic or electrophotographic imaging and printing apparatuses and machines, and more particularly is directed to a two component development system wherein a donor roll is loaded with toner particles that are induction charged.
Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image from either a scanning laser beam, an LED array or an original document being reproduced. By selectively discharging certain areas on the photoconductor, an electrostatic latent image is recorded on the photoconductive surface. This latent image is subsequently developed by charged toner particles supplied by the development sub-system.
Powder development systems normally fall into two classes: two component, in which the developer material is comprised of magnetic carrier granules having toner particles adhering triboelectrically thereto and single component, which typically uses toner only. Toner particles are attracted to the latent image forming a toner powder image on the photoconductive surface. The toner powder image is subsequently transferred to a copy sheet, and finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.
The electrostatic image in electrophotographic copying/printing systems is typically developed with a nonmagnetic, insulative toner that is charged by the phenomenon of triboelectricity. The triboelectric charging is obtained by either mixing the toner with larger carrier beads in a two component development system or by rubbing the toner between a blade and donor roll in a single component system. Although such powder development systems have served the electrophotographic industry well over the years, there continues to be a need for improvements in toner charging since triboelecticity is not well understood and predictable due to a strong materials sensitivity. The materials and environmental sensitivity causes difficulties in identifying a triboelectrically compatible set of color toners that can be blended for custom colors. Furthermore, to enable “offset” print quality with powder based electrophotographic development systems, a small toner (˜5 μm) size is desired. Although the functionality of small triboelectrically charged toners has been demonstrated, concerns remain regarding the long-term robustness of such systems.
The operating latitude of a powder xerographic development system is determined to a great degree by the method with which toner particles are supplied to an electrostatic image. Placing charge on the particles, to enable movement and imagewise development via electric fields, is most often accomplished with triboelectricity. However, all development systems which use triboelectricity to charge toner, whether they be two component (toner and carrier) or mono-component (toner only), have one feature in common: charges are distributed non-uniformly on the surface of the toner. This results in high electrostatic adhesion due to locally high surface charge densities on the particles. Toner adhesion, especially in the development step, is a key factor which limits performance by hindering toner release. As the toner particle size is reduced to enable higher image quality, the charge Q on a triboelectrically charged particle, and thus the removal force (F=QE) acting on the particle due to the development electric field E, will drop roughly in proportion to the particle surface area. On the other hand, the electrostatic adhesion forces for tribo-charged toner, which are dominated by charged regions on the particle at or near its points of contact with a surface, do not decrease as rapidly with decreasing size. This so-called “charge patch” effect makes smaller, tribo-charged particles much more difficult to develop and control.
Jumping development systems, in which toner is required to jump a gap to develop the electrostatic latent image, are capable of image quality which can be superior to in-contact systems, such as magnetic brush Is development. Unfortunately, they are also much more sensitive to toner adhesion. In fact, high toner adhesion has been identified as a major limitation in jumping development. Up now, mechanical and/or electrical agitation of toner have been used to overcome the adhesion forces and allow toner to be released into a cloud for jumping development. This approach has had limited success, however. More agitation often releases more toner, but high adhesion due to triboelectric charging still dominates in toner cloud generation and causes unstable development. For full color printing system architectures in which the complete image is formed on the image bearing member, an increase in toner delivery rate produces a highly interactive toner cloud, which disturbs previously developed particles on the latent image. This erases many of the original benefits of jumping development for color xerographic printing for the so-called image-on-image (IOI) architecture. Again, as the toner size is reduced, the above limitations become even more acute due to increased toner adhesion.
Given that charged particle adhesion is a major limiting factor in development with dry powder, it has been a goal to identify toner charging and delivery schemes which keep toner adhesion low. Clearly, the adhesion of the charged toner depends sensitively on the method used to charge the particles.
To circumvent limitations associated with development systems based on triboelectrically charged toner, non-tribo toner charging systems are desired to enable a more stable development system with greater latitude in toner materials. Conventional single component development (SCD) systems based on induction charging utilize a magnetic loaded toner to suppress background deposition. However, the magnetic material precludes bright colors.
To circumvent these problems, a new electrophotographic development system design has been described in U.S. Pat. Nos. 6,503,678, 6,492,082, and 6,353,723, and U.S. application Ser. Nos. 09/739,405 and 09/724,064 which hereby are incorporated by reference. The patents and applications disclose a single component development system based on the induction charging of conductive, nonmagnetic toner prior to the development zone. The conductive toner can either be electrostatically transferred to an (insulative) intermediate belt and/or rheologically transferred directly to paper.